13 Montero et al.: Photoinhibition of five marine microalgae by solar radiation the P-treatment as for the PA- and PAB-treatments. The same repair mechanisms may thereby have operated in this alga following inhibition of photosynthesis by PAR or UVR. This fact suggests an inhibition threshold in Amphidinium sp. that would have already been surpassed by PAR radiation itself. Time constant for P. tricornutum in the P-treatment was, in contrast, substantially enhanced in comparison to the time constants for the other treatments, thus showing a notable capability for recovery of this alga after inhibition by high PAR. In general, the pattern of recovery time constants of oxygen evolution (s OE ) was somewhat different to the pattern of recovery time constants of F v /F m, in particular for Amphidinium sp. and P. tricornutum. Nonetheless, since oxygen evolution showed a positive dependence on F v /F m with slope > 1, photosynthesis is likely to have been mainly limited by inhibition of the electron flow at PSII in all the algae except, perhaps, in P. tricornutum (slope < 1, Nogués and Baker, 1995). Damage to DNA in the PAB-treatment should also not be ruled out in I. galbana. Cell density was not negatively affected in any of the algae, and increases in cell density at the end of the recovery period were even significant in P. tricornutum and I. galbana. Karentz et al. (1994) also reported no significant differences in growth between exposed and non-exposed samples of phytoplankton to UVR in an exclusion experiment. Therefore, our results are in agreement with those of Villafañe et al. (1995) in that strong inhibition of photosynthesis can not correlate with reduction of cell density, in particular when photoinhibion stems from short-term exposure. Even though not all the algae were affected to the same extent, Chl c and carotenoids underwent a larger reduction than Chl a, this result being in agreement with those previously shown in the literature (Häder and Worrest, 1991; Häberling and Häder, 1992) about the greater susceptibility of Chl c and carotenoids to bleaching than Chl a. The reduction in the Chl c/ Chl a and Car/Chl a ratios may indicate a partial damage to the structure of the photosynthetic apparatus, which may have contributed to photoinhibition in almost all algae, even though only in Cryptomonas sp. it seems to have played a significant role (see above). In conclusion, photoinhibition measured in natural populations is likely to depend on species composition and their relative abundance (Helbling et al., 1992), but our results show that phytoplanktonic species tolerance to UVR is also dependent on the capability for recovery, which may not correlate with the sensitivity to inhibition (Roos and Vincent, 1998). The composition of the photosynthetic apparatus may explain, at least in part, the differential sensitivity between microalgae, but other factors related to efficiency of the repair mechanisms are likely more determinant. Acknowledgements This work was financed by the Spanish Ministry of Education through the Commission of Science and Technology ninguna de las algas, e incluso hubo un aumento significativo de la densidad celular al final del periodo de recuperación en P. tricornutum e I. galbana. Karentz et al. (1994) también mostraron que no había diferencias significativas en el crecimiento entre muestras de fitoplancton expuestas y no expuestas a UVR en un experimento de exclusión selectiva de longitudes de onda. Por lo tanto, nuestros resultados están de acuerdo con los de Villafañe et al. (1995), en el sentido de que una inhibición profunda de la fotosíntesis puede no estar correlacionada con la reducción de la densidad celular, en particular cuando la fotoinhibición se origina por exposiciones breves. Aún cuando no todas las algas fueron afectadas al mismo nivel, la Chl c y los carotenoides soportaron una reducción mayor que la Chl a, estando este resultado de acuerdo con lo mostrado previamente en la literatura (Häder y Worrest, 1991; Häberling y Häder, 1992) sobre una mayor susceptibilidad de la Chl c y los carotenoides a la decoloración que la de Chl a. La reducción de los cocientes Chl c/chl a y Car/Chl a puede ser indicativa de un daño parcial a la estructura del aparato fotosintético, el cual puede haber contribuido a la fotoinhibición en casi todas las algas, aún cuando solamente en Cryptomonas sp. parece haber jugado un papel significativo (ver más atrás). En conclusión, la fotoinhibición medida en poblaciones naturales depende probablemente de la composición de especies y su abundancia relativa (Helbling et al., 1992), pero nuestros resultados muestran que la tolerancia a UVR de las especies fitoplanctónicas depende también de la capacidad de recuperación, la cual puede no correlacionarse con la sensibilidad a la inhibición (Roos y Vincent, 1998). La composición del aparato fotosintético puede explicar, al menos en parte, la diferente sensibilidad entre las microalgas, pero otros factores relacionados con la eficiencia de los mecanismos de reparación son probablemente más determinantes. Agradecimientos Este trabajo fue financiado por el Ministerio de Educación Español a través de la Comisión de Ciencia y Tecnología (CICYT), proyecto AMB CO2-2. Los autores agradecen a José L. Palazón su ayuda en el análisis estadístico. Traducido al español por los autores. (CICYT), project AMB CO2-2. The authors thank Jose L. Palazón for helping with the statistical analysis. References Aráoz, R. and Häder, D.P. (1997). Ultraviolet radiation induces both degradation and synthesis of phycobilisomes in Nostoc sp.: a spectroscopic and biochemical approach. FEMS Microbiol. Ecol., 23: Behrenfeld, M.J., Chapman, J.W., Hardy, J.T. and Lee, H. (1993). Is there a common response to ultraviolet B radiation by marine phytoplankton?. Mar. Ecol. Prog. Ser., 12:

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